Compact large-aperture interferometer and adjusting method

Abstract

A collimating light path of the interferometer comprises a collimating mirror, a first reflecting mirror, a second reflecting mirror, a compensating mirror group, a third reflecting mirror and a spectroscope, and an emitted laser beam sequentially passes through the spectroscope, the third reflecting mirror, the compensating mirror group, the second reflecting mirror, the first reflecting mirror and the collimating mirror to form a laser beam; and after being reflected by the to-be-measured element, the light returns along the same path and enters the imaging mirror. According to the invention, the three groups of reflectors can fold a collimated light path for multiple times, a long baseline is compressed into a limited space, and the size of large-aperture interference is greatly reduced; and meanwhile, an adjustment method is utilized to perform precise adjustment on an interferometer light path system, including detection of parallelism of emergent light, wavefront of the interferometer, wavefront uniformity and the like, so that the measurement precision of the interferometer is improved.

Description

Technical Field

[0001] This invention relates to the field of interferometry, and in particular to a compact large-aperture interferometer and its assembly method. Background Technology

[0002] An interferometer is an optical instrument that uses the principle of light interference to perform high-precision measurements. With the increasing demand for higher resolution, wider dynamic range, and stronger detection capabilities in fields such as high-throughput lithography objectives, astronomical imaging, laser nuclear fusion, and industrial metrology, there is a need for interferometers with larger apertures.

[0003] Large-aperture interferometers can be categorized into vertical and horizontal types based on their mechanical structure. Currently, large-aperture interferometers are created by combining a 4-inch small-aperture interferometer with a beam expander system. However, the beam expander system is extremely large. While a horizontal system can be laid flat on an air-bearing platform, it's clearly unsuitable for a vertical system to be mounted on top of a vertical support. Therefore, a design is needed that minimizes the size and weight of the large-aperture interferometer while maintaining its accuracy, facilitating its use in laboratories, factories, and other similar environments. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a compact large-aperture interferometer and its assembly method, which improves the measurement accuracy of the interferometer while reducing its size.

[0005] Technical solution: To achieve the above objectives, the present invention provides a compact large-aperture interferometer, wherein the collimating optical path of the interferometer includes a collimating mirror, a first reflecting mirror, a second reflecting mirror, a compensating mirror group, a third reflecting mirror, and a beam splitter. The emitted laser beam passes sequentially through the beam splitter, the third reflecting mirror, the compensating mirror group, the second reflecting mirror, the first reflecting mirror, and the collimating mirror. After being reflected by the element under test, it returns along the original path and enters the imaging mirror.

[0006] Preferably, the collimating lens has an aperture of 330mm, a center thickness of 40mm, and a focal length of 1700mm; the compensation lens group includes compensation lens 1 and compensation lens 2, wherein compensation lens 1 has an aperture of 120mm, a center thickness of 15mm, and a focal length of 290mm; and compensation lens 2 has an aperture of 120mm, a center thickness of 13mm, and a focal length of 380mm.

[0007] Preferably, the collimating lens, compensating lens 1 and compensating lens 2 are plano-convex lenses.

[0008] Preferably, the distance along the optical axis between the compensation lens group and the collimating lens is 1350 mm.

[0009] Preferably, the first reflector has a diameter of 330 mm and a thickness of 45 mm; the second reflector has a diameter of 210 mm and a thickness of 25 mm; and the third reflector has a diameter of 150 mm and a thickness of 20 mm.

[0010] Preferably, the collimating lens and compensating lens assembly are made of H-K9L material.

[0011] Preferably, the first, second, and third reflectors are made of Corning 7980, and their reflective surfaces are coated with a protective aluminum film.

[0012] The present invention provides a method for adjusting a compact large-aperture interferometer, comprising: removing the third reflecting mirror; mounting the internal focusing telescope behind the compensating mirror group; adjusting the first and second reflecting mirrors between the collimating mirror and the compensating mirror group to make the collimating mirror and the compensating mirror group coaxial; installing the third reflecting mirror; moving the internal focusing telescope behind the imaging mirror; adjusting the internal focusing telescope according to the optical axis of the imaging mirror; finding the spherical center image of the compensating mirror group through the beam splitter and the third reflecting mirror; and adjusting the beam splitter and the compensating mirror group to make the compensating mirror group coaxial with the imaging mirror.

[0013] Preferably, the assembly process also includes parallelism detection of the emitted light, interferometer wavefront detection, and wavefront uniformity detection.

[0014] Beneficial effects: The present invention has the following advantages: The present invention improves the measurement accuracy of the interferometer while reducing the size of the large-aperture interferometer, and significantly reduces the manufacturing difficulty and cost. Attached Figure Description

[0015] Figure 1 This is a front view of a compact large-aperture interferometer.

[0016] Figure 2 Top view of a compact large-aperture interferometer;

[0017] Figure 3 This is a schematic diagram of the adjustment of a compact large-aperture interferometer;

[0018] Figure 4 A schematic diagram illustrating the adjustment of Seidel aberrations in a compact, large-aperture interferometer. Detailed Implementation

[0019] The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings.

[0020] like Figures 1-2As shown, the collimating optical path of the compact large-aperture interferometer of the present invention includes a collimating mirror, a first reflecting mirror, a second reflecting mirror, a compensating mirror group, a third reflecting mirror, and a beam splitter. The laser inside the interferometer is located below the PBS and is conjugate to the focal point of the imaging mirror of the interferometer. The emitted laser beam passes sequentially through the beam splitter, the third reflecting mirror, the compensating mirror group, the second reflecting mirror, the first reflecting mirror, and the collimating mirror. After being reflected by the element under test, it returns along the same path to the imaging mirror.

[0021] The first, second, and third reflecting mirrors are all plane mirrors, capable of folding the collimating light path multiple times, compressing the long baseline into a limited space. Taking a collimating mirror with a focal length of 1700mm as an example, the collimating light path, after being folded by the three sets of reflecting mirrors, has a length of 750mm. With the collimating mirror facing vertically downwards, the height reaches 650mm, greatly reducing the volume of large-aperture interference. By setting the positions of the three sets of reflecting mirrors in the folded light path, the imaging light path and most photoelectric components, such as the compensating mirror, can be placed horizontally, while the outgoing light path faces vertically downwards through the collimating mirror, meeting the light output requirements of a vertical interferometer.

[0022] The first, second, and third reflecting mirrors are fused silica plane mirrors, polished using ion beam or magnetorheological methods, with a surface flatness ≤λ / 40 (PV value) and a local slope error <0.5urad. They are also coated with a protective aluminum film (reflectivity >98%). Under these process requirements, the optical path folding design has less than λ / 10 (PV) of impact on the entire collimation system. In this implementation, the specific parameter design of the three sets of reflecting mirrors is as follows:

[0023] First reflecting mirror: 330mm in diameter, 45mm in thickness, made of Corning 7980 material;

[0024] Second reflector: 210mm in diameter, 25mm in thickness, made of Corning 7980 material;

[0025] Third reflecting mirror: 150mm in diameter, 20mm in thickness, made of Corning 7980.

[0026] This invention employs a large-aperture collimating lens and a small-aperture compensating lens assembly to optimize imaging performance. Furthermore, placing the compensating lens assembly between the second and third reflecting mirrors not only reduces the diameter of the compensating lens assembly, lowering costs, but also simplifies assembly and adjustment. The collimating lens is a plano-convex mirror, handling over 80% of the optical power. The compensating lens corrects residual images (primarily compensating for 80% spherical aberration and 15% coma), improving wavefront error from λ / 5 to λ / 10 (RMS). In this embodiment, the distance along the optical axis between the compensating lens assembly and the collimating lens is 1350 mm. The specific parameter design of the collimating lens and the compensating lens assembly is as follows:

[0027] Collimating lens: 330mm aperture, 40mm center thickness, 1700mm focal length, material is H-K9L;

[0028] Compensation lens 1: 120mm aperture, 15mm center thickness, 290mm focal length, material is H-K9L;

[0029] Compensation lens 2: 120mm diameter, 13mm center thickness, 380mm focal length, material is H-K9L.

[0030] During optical path debugging, the compensation mirror assembly is used for assembly and adjustment assistance, providing sensitive adjustment freedom (with six-dimensional adjustment capability, translation ±5mm, resolution 1µm, tilt ±1°, resolution 1″; every 10µm displacement corresponds to 0.05λ wavefront change, every 1′ tilt corresponds to 0.1λ coma, and the compensation mirror can be precisely adjusted using an interferometer or wavefront sensor).

[0031] This invention provides a method for assembling the aforementioned compact large-aperture interferometer:

[0032] 1. Tool preparation: high-precision six-dimensional adjustment frame, laser, internal focusing telescope (adjustment accuracy: ±0.4″), shearing interferometer, small interferometer for detection or Shaker-Hartmann wavefront sensor, etc.

[0033] 2. Environmental requirements: The laboratory temperature is 21℃±0.2℃, the humidity is <50%, it is a Class 1000 laboratory, and the equipment is placed on an air-floating vibration isolation platform;

[0034] 3. Utilizing the linearity of the laser, and in conjunction with the aperture stop, adjust the spatial position of all lenses and the angle and position of the reflectors to be basically coaxial, with an adjustment accuracy of ±0.5′ and ±1mm;

[0035] 4. First, remove the third reflecting mirror. Then, mount the internal focusing telescope behind the compensating mirror group. Adjust the first and second reflecting mirrors between the collimating mirror and the compensating mirror group to ensure they are coaxial, with an error within 1″. Figure 3 As shown, the specific adjustment process is as follows:

[0036] 1) Use an internal focusing telescope to find the spherical center images of the two surfaces of the compensating lens group respectively;

[0037] 2) Adjust the spatial position of the internal focusing telescope so that the internal focusing telescope and the compensating lens group are coaxial;

[0038] 3) Use an internal focusing telescope to locate the two spherical centers of the collimating lens;

[0039] 4) Adjust the first and second reflecting mirrors so that the collimating mirror and the compensating mirror group are coaxial (preferably, the two reflecting mirrors control the positions of the two spherical centers respectively).

[0040] 6. Install the third reflecting mirror, move the internal focusing telescope behind the imaging mirror, and adjust the internal focusing telescope according to the optical axis of the imaging mirror;

[0041] 7. The internal focusing telescope uses the beam splitter and the third reflecting mirror to find the spherical center image of the compensating mirror group. By adjusting the beam splitter and the compensating mirror group, the compensating mirror group and the imaging mirror are made coaxial.

[0042] 8. Parallelism detection of outgoing light: The divergence angle of the outgoing beam from the interferometer should be less than 10 arcsec. Use a shearing interferometer or pentaprism scanning method to detect the parallelism of the outgoing light. Methods to adjust the parallelism include: adjusting the front and back positions of the compensating mirror group, fine-tuning the angle of the compensating mirror group or the reflecting mirror, and adjusting the front and back positions of the lens behind the cubic prism PBS.

[0043] 9. Interferometer Wavefront Detection: The interferometer wavefront error must be ≤λ / 2 PV. Place a small interferometer or a Shaker-Hartmann wavefront sensor behind the imaging mirror and directly read the interferometer wavefront error and Seidel aberration. Figure 4 As shown. Based on the Seidel aberration guidelines for spherical aberration, coma, and astigmatism, fine adjustments are made to minimize each aberration as much as possible, until the combined wavefront error is ≤ λ / 2 PV.

[0044] Spherical aberration is adjusted by controlling the parallelism of the outgoing light; coma is usually caused by the components not being on the same optical axis, and is adjusted by designing compensation terms, such as adjusting the eccentricity of the compensating lens group, adjusting the eccentricity of the imaging lens, or finely adjusting the mirror until the coma is minimized; astigmatism is usually a comprehensive manifestation of the irregular surface shape of each optical element, and is adjusted by controlling the surface shape, assembly stress, and material uniformity of each optical element, and can also be found by rotating the optical element (such as the mirror) to find the minimum azimuth angle of astigmatism.

[0045] 10. Wavefront uniformity detection: ≥70%. The wavefront uniformity of the interferometer can be measured by the interferometer camera, which measures the ratio of edge energy to center energy. This is mainly controlled by filtering the laser source. The surface shape of optical components, material uniformity, and the accuracy of filter adjustment during assembly will affect the wavefront uniformity.

[0046] 11. After assembly, the accuracy and repeatability of the entire interferometer system are measured: repeatability is required to be ≤λ / 10000 RMS; the accuracy of the three-plane absolute measurement is better than λ / 100 PV. Accuracy and repeatability are mainly affected by a series of factors such as the structural strength and stability of the interferometer, the measurement environment (vibration, temperature), the stability of the light source, and the accuracy of the reference plane and the measured plane.

Claims

1. A compact large-aperture interferometer, characterized in that, The collimating optical path of the interferometer includes a collimating mirror, a first reflecting mirror, a second reflecting mirror, a compensating mirror group, a third reflecting mirror, and a beam splitter. The emitted laser beam passes sequentially through the beam splitter, the third reflecting mirror, the compensating mirror group, the second reflecting mirror, the first reflecting mirror, and the collimating mirror. After being reflected by the element under test, it returns along the original path and enters the imaging mirror.

2. The compact large-aperture interferometer according to claim 1, characterized in that, The collimating lens has an aperture of 330mm, a center thickness of 40mm, and a focal length of 1700mm; the compensation lens group includes compensation lens 1 and compensation lens 2, wherein compensation lens 1 has an aperture of 120mm, a center thickness of 15mm, and a focal length of 290mm; compensation lens 2 has an aperture of 120mm, a center thickness of 13mm, and a focal length of 380mm.

3. The compact large-aperture interferometer according to claim 2, characterized in that, The collimating lens, compensating lens 1, and compensating lens 2 are all plano-convex lenses.

4. The compact large-aperture interferometer according to claim 1, characterized in that, The distance along the optical axis between the compensating lens group and the collimating lens is 1350 mm.

5. The compact large-aperture interferometer according to claim 1, characterized in that, The first reflector has a diameter of 330 mm and a thickness of 45 mm; the second reflector has a diameter of 210 mm and a thickness of 25 mm; and the third reflector has a diameter of 150 mm and a thickness of 20 mm.

6. The compact large-aperture interferometer according to claim 1, characterized in that, The collimating lens and compensating lens assembly are made of H-K9L material.

7. The compact large-aperture interferometer according to claim 1, characterized in that, The first, second, and third reflectors are made of Corning 7980, and their reflective surfaces are coated with a protective aluminum film.

8. A method for adjusting the compact large-aperture interferometer as described in claim 1, characterized in that, include: Remove the third reflecting mirror and mount the internal focusing telescope behind the compensating mirror group. Adjust the first and second reflecting mirrors between the collimating mirror and the compensating mirror group to make the collimating mirror and the compensating mirror group coaxial. Install the third reflecting mirror and move the internal focusing telescope behind the imaging mirror. After adjusting the internal focusing telescope according to the optical axis of the imaging mirror, use the internal focusing telescope to find the spherical center image of the compensating mirror group through the beam splitter and the third reflecting mirror. Adjust the beam splitter and the compensating mirror group to make the compensating mirror group coaxial with the imaging mirror.

9. The mixing method according to claim 8, characterized in that, The assembly process also includes parallelism detection of the emitted light, interferometer wavefront detection, and wavefront uniformity detection.

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